FIELD OF THE INVENTION
The invention relates to a fuel-cell system, in particular a high-temperature fuel-cell system, including at least one fuel-cell block having an anode section and a cathode section. The invention also relates to a method for operating such a fuel-cell system.
A fuel cell contains an anode and a cathode which are separated by an immediately adjacent, ion-conducting electrolyte. The electrolyte may be composed of an ion-conducting liquid, of a polymer membrane or, as in the case of a high-temperature fuel cell, of a solid body, such as of zirconium oxide containing small additions of yttrium oxide, for example. The electrolyte of a high-temperature fuel cell is oxygen-ion-conducting at operating temperatures of the high-temperature fuel cell of about 1000.degree. C. The fuel, generally hydrogen, is fed to the anode, oxygen or combustion air is fed to the cathode by suitable duct systems and water produced in the reaction of hydrogen and oxygen is discharged from the fuel cell with anode waste gas or cathode waste gas, depending on the type of fuel cell. A fuel cell can convert the fuel into electrical energy with a higher efficiency and lower environmental pollution than is capable of being done by conventional, heretofore-known internal-combustion engines having an efficiency which is limited by the so-called Carnot process.
In development projects that are currently running, attempts are additionally also being made to utilize the heat produced during the operation of fuel cells, in particular during the operation of high-temperature fuel cells. Thus, the development of a high-temperature fuel-cell power station proceeds as a rule from the combination of high-temperature fuel cells with a gas turbine and, optionally, a steam turbine connected downstream of the gas turbine, wherein the high-temperature fuel cell takes over the function of the combustion chamber of the gas turbine.
In particular, in devices disclosed in a paper by Dr. E. Erdle entitled "Hochtemperaturbrennstoffzelle SOFC-Stand der Forschung fur eine neue Technik zur Stromerzeugung" High-Temperature SOFC Fuel Cell--State Of Research Aimed At A New Technology For Current Generation! in VDI Berichte No. 1029, 1993 and in German Published, Non-Prosecuted Patent Application DE 40 21 097 A1, for operating a high-temperature fuel cell, a bifurcation is provided in each case for the cathode waste gas produced on the cathode side, at which bifurcation a first portion of the cathode waste gas is fed to a combustion chamber and at which bifurcation a second portion of the cathode waste gas is conveyed through a recuperative heat exchanger to the temperature-lowering system and then mixed with an inflow of cooler air. The inflow of cooler air is fed back into the cathode gas spaces together with the cooled subflow of the cathode waste gas through a compressor and the same recuperative heat exchanger. The non-recirculated portion of the cathode waste gas is combusted with the anode waste gas in a burner. The flue gas of that combustion process is normally fed to a gas turbine. That configuration is quite sensible thermodynamically, but has the disadvantage of requiring the cathode waste gas to be bifurcated at about 1000.degree. C. Consequently, a high expenditure is necessary in regard to the pipe system and the connecting and welding procedure.
A further disadvantage of that configuration is that the amount of recirculated cathode waste gas must not exceed a certain proportion because the quantity of heat might otherwise no longer be adequate to preheat the cathode gas to be fed to the fuel cell.